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Old Sep 5, 2009, 9:44 AM   #1
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Default 20x30 cm photo

Good afternoon to everyone,
I want to make a photo of 20x30 cm and 300 ppi , jpg format.

I have tried it with irfanview program...but when i set the first parameter ex 30 the other appears automatically and i cant write it.

So which program do you advise me to use exept PS gos i dont have .
Thank you very much
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Old Sep 5, 2009, 9:55 AM   #2
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How large is the original image (dimensions in pixels)?

20 x 30cm would be roughly 8x12 inches

So, if you wanted 300 pixels per inch, you'd need an image size of 2400 x 3600 pixels or 8.64 Megapixels (300 pixels x 8 inches = 2400 pixels; 300 pixels x 12 inches = 3600 pixels).


You can use Irfanview to resize an image if you want to do that (look under Image>Resize/Resample). I normally use the Lanczos algorithm for resizing. You'd want to crop the image to the correct aspect ratio (ratio of width to height) first.

But, if you are starting with an image that's too small, you can introduce degradation by resizing (or "uprezzing") it, because you're adding pixels that were not captured by the camera (based on the values of adjacent pixels). If the image is already large enough (or larger than needed for the desired pixels per inch), then I would not downsize it to match (the printer driver will do that automatically for you).

Also note that 300ppi is not necessarily the best choice, depending on the printer.

If you want a more sophisticated product for printing (that automatically optimizes the ppi for the printer driver being used for best results using sophisticated interpolation algorithms), try QImage Pro. You can download a trial version to see if you like it.

http://www.ddisoftware.com/qimage/
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Old Sep 5, 2009, 10:57 AM   #3
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firsty thanks for your direct reply.
these rules(300ppi) are for a contest...
i have 2 photos in different dimension....3456x2592 and 3072x2304 .from what i understand the first is > 300ppi and the 2nd <<.
so what shall i do first??
crop them?? and then???and with which program ?
Can QImage pro solve this matter?
Iv allready resized them with infanview.


thank you
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Old Sep 5, 2009, 11:05 AM   #4
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They are already the correct Aspect Ratio, so you don't need to crop them. If you need a photo that will print at exactly 300ppi, then you just need to resize them to the correct dimensions in width x height (which should work out to around 2400 x 3600 pixels for that print size). But, I wouldn't do that unless they require it (as you can degrade image quality by resizing).
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Old Sep 5, 2009, 11:27 AM   #5
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Note that you can do the same thing by the desired print size and dpi under Image>Resize/Resample. Just plug in the dimensions in cm and plug in 300 in the dpi box. It looks like 20x30cm will be slightly smaller in dimensions than I calculated for 8x12" if you use the exact size.

Is that what you said wasn't working the first time? It should work OK (I just tried it and it works fine for me).
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Old Sep 5, 2009, 5:50 PM   #6
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Well i tried it too many times with irfanview and still have a problem.when i set the fisrt parameter the other changes by herself and vice versa.
well i install adobe photoshop elements...i open the photo--go to resize-image size-i put into resolution 300 pix/inch and tried to write 20 to width and 30 to height.and its appears the same problem with irfan.

what do i do wrong ?? i tried it with the choice resambles image , selected and not.
thanks for helping.
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Old Sep 5, 2009, 6:00 PM   #7
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If you leave the retain aspect ratio box checked with the Resize/Resample menu in Irfanview, and set one side to 30cm, the other should change to something very close to 20cm, as long as the aspect ratio (ratio of width to height) for the orignal image is correct (and if not, you could always crop it a bit if needed). A few pixels difference really isn't going to hurt anything, and you will see minor differences between cameras. The aspect ratio of the image sizes you posted should work fine (they're close enough -- within a few pixels either way of being right on for the aspect ratio needed for that size print).

For example, if you use 30cm x 20cm, then set the dpi to 300, all it's doing is automatically calculating the correct image size (width x height in pixels) for that print size and letting you resize the image dimensions in pixels to match it (just as if you calculated the size manually without using those fields and just resized by pixels instead).

Again, if it's a few pixels off when if you use the retain aspect ratio box (so that other parameters change automatically), it's not going to hurt anything.
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Old Sep 5, 2009, 6:11 PM   #8
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For example, I just loaded an image with an original size of 3600 x 2695 pixels.

If I go into the Resize/Resample menu and resize by cm, then plug in 30cm and 300 dpi while leaving the Preserve aspect ratio box checked, it automatically changes the other side to 19.96cm and uses a new image size of 3543 x 2547 pixels.

That's close enough (about as close as you'd get it without cropping a few pixels off on one side or the other). You don't need it any closer than that. ;-)

But, unless someone really wanted that exact size (representing exactly 300 dpi), I wouldn't have even bothered to resize that image (it was already 3600 x 2395 pixels to begin with, which is pretty close to 300dpi at an 8x12" print size (which is slightly larger than 30cm x 20cm). I suspect they mean "at least" 300dpi versus exactly 300dpi anyway. ;-)
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Old Sep 5, 2009, 9:04 PM   #9
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Oh...thank you so much...you really helped me a lot...and i do have a clear idea of how it really works.
thank you very much!!!
have a nice day!!!
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